A. Field of the Disclosure
The present disclosure relates generally to flood control. More specifically, the disclosure relates to the control of the discharge of flood waters from a source body of water.
B. Background
Controlling the effects of flood events has long been an engineering challenge. The development of agricultural areas or wild areas inevitably changes the patterns of runoff into streams during rainfall events. During the course of development permeable soil is covered with buildings and roads, which are impermeable to water. As a result, the flow of water that hits the ground as precipitation is not slowed by percolation into the ground. This causes runoff to enter rivers and streams at very high rates of volumetric discharge in developed areas. The attendant runoff problems can cause flooding of downstream areas, erosion, and non-point source pollution of rivers and streams.
The conventional approach to controlling runoff from precipitation events is to provide a static flood control structure. In the absence of flood control structures, runoff from a developed area will usually occur at an extremely high initial rate (“peak flow”) and then decline abruptly. Static flood control structures, such as dams and weirs, reduce the peak flow rate and cause the flow to taper off slowly.
Static control structures typically consist of a fixed opening that is designed to restrict the discharge flow rate for a hypothetical design storm event. Of course, a design storm event will never occur. Design storms are based on statistical data compiled over the course of several years from weather stations and other sources. One commonly used process for developing the design storm events is described in a document published by the United States Department of Commerce prepared for the Natural Resources Conservation Service, often referred to as Technical Paper 40 (TP40), published May 1961. This document is widely used for controlling storm water runoff for post-developed watersheds.
Although static controls reduce the impacts of development significantly, they do not emulate the natural pre-development flow patterns in response to precipitation. The standard of practice in site development is to restrict the post-developed peak flow rate to pre-developed peak flow rate conditions for a hypothetical design storm event. This results in gradual decline in the recession limb of the hydrograph consequently resulting in higher discharge velocities and higher discharge flow rates over the duration of the storm event.
Natural flow patterns are more desirable than the types of less attenuated patterns that static control structures provide. In addition to reducing erosion, downstream flooding, and non-point source pollution, natural flow patterns serve to maintain waterlogged (hydric) soils that form and maintain wetlands. The wetlands in turn mitigate floods, remove pollutants, and spawn wildlife. Static control structures cannot emulate natural flow. The natural flow patterns from a given drainage area are the result of many complex interacting factors, such as heterogeneous soil porosity, the presence (or absence) of impermeable soil layers, and topography. Furthermore, natural flow patterns will vary with the severity of a precipitation event. Current control structure designs are generated from idealized “design storm events.” Design storm events are created from hypothetical methods calculated from historical data. These data, being generated by mathematical models, may not reflect the variations of depth, intensity, and durations of actual precipitation events. Furthermore, the rate of discharge of static control structures is a direct function to the hydrostatic pressure (also known as “head,” which is directly related to depth) in the detention body, making it impossible to replicate pre-developed hydrology.
Consequently there is a need in the art to provide natural flow patterns even after areas have been developed.